User guide

The threshold voltage can be derived from the input signal via a low-pass filter.
This will work fine for most applications. But applications that do not provide a
continuous data stream at the input (for example, any application using bursts)
cannot use this low-pass filter, because the threshold voltage will drift from the
correct level when there is no input. In such cases, the threshold can be specified
manually. It is then no longer derived from the input signal (see the following
figure). The manually set threshold voltage must of course be within the input
range.
The difference between the data path and the CDR path is that the comparator of
the CDR is always single-ended. Thus, this comparator always needs a threshold
voltage that lies between the high and low levels of the incoming signal.
The differential threshold of the data path comparator has no relation to the single-
ended threshold of the CDR path comparator. This means that in differential mode,
the two thresholds will be different and in single-ended mode (either normal and
complement) they will/can be equal (except during measurements).
The following figure shows a simplified block diagram. It does not reflect the
different input modes (especially the differential case), but it matches both single-
ended cases.
Low Pass
Filter
Threshold
Voltage
Threshold
Voltage
DC tracking on/off
switches
Input
Connector
Comparator
Comparator
CDR
Data
Processing
recovered
Clock
Bit Rate Range
The Serial BERT provides bit rates from 150Mbit/s up to 12.5 Gbit/s, depending
on the instrument's options.
However, several specific properties and limitations need to be taken into account
when working at low bit rates. The limitations apply to the instrument according
to the following hysteresis curve:
• If the bit rate falls below 615 Mbit/s, the limitations apply.
• If the bit rate exceeds 620 Mbit/s, the limitations no longer apply.
5 Setting up the Error Detector
176 Agilent J-BERT N4903B High-Performance Serial BERT